The present invention pertains to improvements in the field of electrocoagulation printing. More particularly, the invention relates to an improved electrocoagulation printing ink.
In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The dispersion containing the olefinic substance and the metal oxide is applied onto the surface of the positive electrode in a manner so as to form on the electrode surface micro-droplets of olefinic substance containing the metal oxide. As described in the aforementioned patent, this may be achieved by means of a device comprising a rotatable brush provided with a plurality of radially extending horsehair bristles having extremities contacting the electrode surface, and a distribution roller arranged in spaced-apart parallel relation to the brush such as to contact the bristles thereof at their extremities. The distribution roller has a plurality of peripheral longitudinally extending grooves and is partially immersed in a bath containing the dispersion. As the distribution roller rotates in the dispersion, the grooves are filled with the dispersion which is thus transferred to the bristles to coat the extremities thereof. Rotation of the brush, on the other hand, causes the coated bristles to transfer the dispersion onto the surface of the positive electrode and thereby form the desired micro-droplets of olefinic substance containing the metal oxide. Instead of a brush, use can be made of a roller provided with a plurality of radially extending strips of chamois leather adapted to contact the electrode surface, the strips being coated in the same manner as the bristles. Rotation of such a roller causes the coated strips to impinge upon the surface of the positive electrode such as to transfer thereon the dispersion and thereby form the desired micro-droplets of olefinic substance containing the metal oxide.
The electrocoagulation printing ink which is used to fill the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. The coloring agent can be a dye or a pigment. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated colloid adhered to the surface of the positive electrode.
The colloid generally used is a linear colloid of high molecular weight, that is, one having a weight average molecular weight comprised between about 100,000 and about 1,000,000. Examples of suitable colloids include natural polymers such as albumin, gelatin, casein and agar, and synthetic polymers such as polyacrylic acid, polyacrylamide and polyvinyl alcohol. When the colloid used is an anionic copolymer of acrylamide and acrylic acid, Applicant has observed that the dots of coagulated colloid produced upon electrocoagulation of such a colloid are easily scored by the rubber squeegee used to remove any remaining non-coagulated colloid from the surface of the positive electrode, thereby adversely affecting the quality of the image reproduced by electrocoagulation. In addition, since water is generally used as dispersing medium and since surfactants are generally associated with the aforesaid acrylamide copolymer to assist in dispersing the copolymer to provide the desired colloidal dispersion, the surfactants cause the colloid to retain water so that the residual non-coagulated colloid remaining on the surface of the positive electrode does not dry rapidly enough to be removed by the rubber squeegee and thus is transferred onto the substrate, thereby producing undesirable background on the electrocoagulation printed images.